This invention relates to satellite communications systems using multiple spot beams to selectively broadcast high bit rate broadband information to user terminals located within desired coverage areas and, more particularly, to a satellite communications system in which a high bit rate broadband data stream is divided into multiple data streams in a hub for delivery to user terminals located within desired coverage areas via a multiple-transponder satellite.
Satellites are used extensively for a variety of communications applications as a result of some well-recognized benefits. The most important communications advantage that satellites enjoy is that they are in view of a large amount of the earth""s surface. A geosynchronous satellite is in view of about one-third of the earth""s surface, for example.
In addition, large amounts of frequency spectrum have been allocated to satellites for communications in the microwave and millimeter wave frequencies. For example, at C and Ku bands, the available spectrum for satellite communications is on the order of one GHz. That bandwidth can be made available to users located in the field of view of the satellite and can be multiplied through a variety of frequency reuse techniques.
Moreover, because satellite communications are carried out using radio frequencies through free space, mobile user terminals can be deployed. At the common satellite communications frequencies of C and Ku band, reasonably sized antennas and low cost user terminal hardware are readily available.
The advantages of satellite communications have been enjoyed by users in several types of applications. For example, cable TV networks deliver high bandwidth video programming to head-end distribution points around the world, and national retailers use VSAT networks to accept and distribute data to and from retail stores throughout the country.
The first applications of communications satellites were in point-to-point communications links between fixed pairs of earth stations 12. As shown in FIG. 1, communications from one earth station to the other is via a dedicated path over the satellite 13. As a result, two links are needed in order to provide full duplex communications. Before the advent of underseas fiber optic cable, many international telephone calls were accomplished with point-to-point satellite links.
Today, satellites are more commonly used for point-to-multipoint, or broadcast, applications as illustrated in FIG. 2. In FIG. 2, the information to be broadcast to a number of receivers within the field of view of the satellite is delivered to a hub 14 which then uplinks the information to the satellite 13. The satellite then relays the information to user terminals 15 located within a broad coverage footprint 16 on the earth. For example, television programming from a single network hub can be delivered to numerous ground-based broadcasters or cable operators by a geosynchronous satellite. Such broadcast applications take full advantage of the wide area coverage provided by geosynchronous satellites. Every television station in the United States owns and operates at least one broadcast receive terminal, and many stations own uplink terminals to deliver news feeds via satellite. Television programming is most commonly distributed using the C band of frequencies.
Today, roughly one-third of all satellite transponders are dedicated to the distribution of cable television programming. A typical cable system head-end in the United States will continuously receive 30 to 50 satellite-delivered video channels coming from several different satellites. To control access by cable subscribers to the individual channels, the programming is typically encoded at the uplink using standards developed by the Motion Picture Experts Group (MPEG) or according to one of the digital video broadcasting (DVB) standards. The receiving site at the cable system head-end then requires a decoder for each channel being recovered.
Direct-to-home video distribution systems such as the Hughes DirecTV system typically employ high-power Ku band satellites using the BSS channel plan. Under that plan, each orbit position has 32 channels assigned to it. A satellite might transmit 16 channels at one time, thus requiring that two satellites be operated in the same slot. The full complement of 32 channels of about 27 MHz each can deliver between 150 and 250 compressed digital video channels. In the late 1990s, the DirecTV system and other direct-to-home systems were augmented with data broadcasting to provide Internet access. In such systems, the data is typically transmitted in packets to allow the information to be addressed to individual receivers.
Various schemes for broadcasting digital data are known. In one analog technique, a low data rate data stream is inserted into the vertical blanking interval of the video transmission. The data is then removed from the video signal by a special decoder unit connected to the display terminal. In another known approach for digital data broadcasting, a medium data rate data stream is modulated onto a baseband subcarrier, and the receiving user terminal recovers the data with a subcarrier receiver and decoder. These systems suffer from bandwidth limitations and from the need for special receivers.
The satellite communications industry has long used the term xe2x80x9ctransponderxe2x80x9d to refer to a defined RF channel of communications within a satellite communications system. A satellite transponder is essentially a microwave relay channel, taking into account the need to translate the frequency of the transponder from an uplink frequency to a downlink frequency. A transponderized satellite payload design breaks up the full downlink frequency band (for example, a 500 MHz band at C band or a 1 GHz band at Ku band) to allow more effective power amplification of the downlink signal by the satellite downlink transmitter. If the transmitter was instead required to accommodate the full bandwidth of the downlink, that requirement would greatly limit the RF output power level available from the transmitter. By dividing the fulldownlink bandwidth into several transponders, or channels, individual amplifiers dedicated to each transponder segment of the downlink (for example, 36 MHz or 54 MHz segments) can be employed and the power level of the full-bandwidth downlink signal can therefore be significantly higher.
In a typical transponderized satellite communications system, the frequency plan of the transponders is coordinated between and among the user terminals, the hub, and the satellite payload. As a result, the information to be relayed over a conventional communication satellite is limited by the bandwidth of a single transponder (which may be 27 MHz, 36 MHz, 54 MHz, for example). As a result, the amount of data that can be delivered to a single receiving terminal through a conventional communications satellite is inherently limited by transponder bandwidth.
The first geosynchronous communications satellites had uplink and downlink coverage footprints that were coincident with the earth field of view or with major continents in view of the satellite. Antennas for creating full earth coverage patterns are fairly simple and use fairly simple feed horn structures. More recently, switchable spot beam antennas have been developed for satellite communications applications to enable the reuse of the uplink and downlink frequencies across a geographic area. For example, if the United States is divided into multiple spot beam coverage areas, as illustrated in FIG. 3, the full frequency range can be reused in each spot beam coverage area to direct different information to different spot coverage areas. One conventional system for broadcasting information from a communications satellite to user terminals located within a plurality of spot beams is illustrated in FIG. 4.
Spot beams are also employed to deliver more RF power over a smaller spot coverage area in order to reduce the size of the receive antenna required for a user terminal to receive information at a given rate. For example, certain classes of user terminal have been widely deployed for applications such as DirecTV or VSAT, and systems that desire to take advantage of that large installed base can use a spot beam antenna to deliver information to those terminals without requiring a user terminal upgrade. Because of transponder bandwidth limitations, however, there is an inherent limitation on the amount of bandwidth that can be delivered to the terminal that cannot be overcome solely by the use of spot beams. Essentially, the data rate to be delivered to any user terminal within any spot beam is limited by the bandwidth of an individual transponder.
In summary, then, the bandwidth limitations of power amplifiers used in satellite downlink transmitters have resulted in the development of the transponder approach, in which the available RF spectrum is divided into manageable channels, or transponders. The data rate at which data can be delivered over any one transponder is necessarily limited by the bandwidth of the transponder itself. While the use of spot beams has developed in order to take advantage of the existing base of small, low cost user terminals, users in those spot beam coverage areas are still limited in their ability to receive broadband data by the bandwidth of an individual transponder. Known satellite communications systems operating in a broadcast mode therefore suffer from limitations on the data rate at which information may be delivered to user terminals.
Known systems for delivering high bandwidth multimedia content to users at high data rates suffer from a number of other limitations as well. The Geocast system being developed by Geocast Network Systems, Inc. promises to deliver high quality multimedia content to personal computer desktops by using new digital television broadcast spectrum. In the Geocast system, broadband content will be uplinked to a Geocast satellite that will in turn broadcast the content to local TV broadcast stations. Each local station will then broadcast the data over digital television frequencies for receipt by users with specially designed receivers. The special receivers can accept live data feeds or can store content for later retrieval. The Geocast system therefore in principle allows users to overcome the bandwidth limitations of conventional Internet connections, for example. After users customize their receivers to their own interests, preferences, and demographics, the Geocast system matches content to individual receivers and delivers matched content to the receivers for real-time or later viewing.
In principle, then, the Geocast system combines the bandwidth and immediacy of broadcast television with the customization and control enabled by web browsing. Unfortunately, however, the Geocast system suffers from several limitations. First, the special digital television receivers required at each user terminal are expensive. In addition, the bandwidth of Geocast delivery service is limited by the bandwidth of the digital television frequencies.
In summary, then, there is no satisfactory existing solution to the problem of delivering multimedia content in a broadcast mode to users at very high data rates. The present invention provides an advantageous solution to this problem by dedicating multiple transponders to a particular data stream so that data can be relayed at very high data rates using a communications satellite that employs a conventional transponder frequency assignment scheme.
The present invention enables the delivery of information to users at very high data rates. The invention provides a broadband multicasting system for distributing information at very high data rates (on the order of 500 Mbps, for example) to user terminals located within a satellite downlink spot beam. According to one aspect, the multicasting system can include a hub that has a transmitter that accepts a serial data stream, converts the serial data stream into parallel data streams, and, using individual modulators, modulates respective uplink transponder signals with the parallel data streams. A hub antenna directs the modulated uplink transponder signals through free space.
The multicasting system can further include an earth-orbiting communications satellite that comprises an uplink receive antenna to receive the uplink transponder signals from the hub, a repeater adapted to translate the uplink transponder frequencies to downlink transponder frequencies, and an antenna adapted to direct the downlink transponder signals to the desired coverage area(s) on the earth.
A multicasting system according to the invention could enable several new business and consumer communications applications. For example, an entire day""s line-up of prerecorded video programming could be delivered during the previous night, making it possible for consumers to create personalized viewing schedules. Web content could be delivered to geographically-dispersed content caching servers to circumvent Internet congestion and reduce web page delivery times. Updates to software and data could be delivered quickly to distributed servers run by software application service providers and electronic commerce service providers.
According to another aspect of the invention, a method for delivering broadband data to users is provided that includes the steps of separating a broadband serial data stream into parallel data streams and modulating each of the parallel data streams onto respective uplink transponder signals. The method further includes the steps of transmitting the uplink transponder signals to an earth-orbiting satellite, downconverting the uplink transponder signals to respective downlink transponder signals, and transmitting the downlink transponder signals to a coverage area on the earth.
According to yet another aspect of the invention, a user terminal is provided that receives data over multiple transponders transmitted by a communications satellite. The user terminal is located in the desired coverage area on the earth that are produced by an antenna on the communications satellite. The terminal can include a signal splitter that separates transponder signals received from the communications satellite, receivers that demodulate the respective transponder signals to produce a plurality of respective parallel data streams, and a buffer adapted to convert the parallel data streams into a very high data rate serial data stream. The user terminal may employ a small antenna and other low-cost components that have been developed for mass-market high power direct broadcast satellite applications.
According to still another aspect of the invention, a method for receiving broadband information from a multicasting satellite at a user terminal located within the desired coverage area of the satellite is provided. The method includes the steps of separating downlink transponder signals, including subsets of the broadband information, from the multicasting satellite. The method further includes the steps of demodulating each of the separated downlink transponder signals to produce respective parallel data streams and buffering the parallel data streams to produce a serial data stream that contains the broadband information.
According to still another aspect of the invention, a method for receiving broadband information from a multicasting satellite at a two-way interactive user terminal located within the desired coverage area of the satellite is provided. The method includes the steps of separating downlink transponder signals, including subsets of the broadband information, from the multicasting satellite. The method further includes the steps of demodulating each of the separated downlink transponder signals to produce respective parallel data streams and either buffering the parallel data streams or directly combining the parallel data streams to produce a serial data stream that contains the broadband information. The terminal may include a return channel transmitter that transmits a return channel signal back through the satellite for reception at the hub.
The invention provides significant improvements over prior art satellite communications systems by enabling the broadcast distribution of data at very high data rates to widely-deployed user terminals. Multiple transponders are dedicated to a particular data stream so that data can be relayed at a very high data rate by a communications satellite that employs a standard transponder frequency assignment scheme. The satellite multicasting system and method according to the present invention thus provide the ability to broadcast data at a very high data rate to multiple users located within a desired coverage area. According to one embodiment of the invention, assigning many or all of the satellite transponders to deliver this broadband data includes the coordination of modulation scheme and parallel data serialization between an uplink hub and the user terminals.